This Article uncovers the forgotten complex of relationships between the U.S. Constitution, citizenship and the laws of war. The Supreme Court today believes that both noncitizens and citizens who are military enemies in a congressionally-authorized war are entitled to judicially-enforceable rights under the Constitution. The older view was that the U.S. government’s military actions against noncitizen enemies were not limited by the Constitution, but only by the international laws of war. On the other hand, in the antebellum period, the prevailing view was U.S. citizenship should carry with it protection from ever being treated as a military enemy under the laws of war. This Article documents how this antebellum understanding about the protection of U.S. citizenship was challenged and overthrown during the first years of the Civil War. As articulated by Union statesmen, members of Congress, lawyers, soldiers and publicists, the rebels by seceding and seeking to throw off their allegiance to the United States and its Constitution, had forfeited their right to be protected by the Constitution. Henceforth, all military actions against them would be governed only by the loose standards of the international laws of war - the standards always applicable to foreign enemies. But if, at its option, the United States chose at times to deal with the rebels not as military enemies but as wayward citizens committing civil crimes like treason, then these citizens retained their pre-war constitutional entitlements. Thus the way the United States choose to respond to the rebels determined the applicable legal regime - whether the Constitution and other municipal protections would apply, or only the harsh laws of war. Starting in 1863 in the Prize Cases, and continuing until the end of the century, the Supreme Court decided over 300 cases arising out of the war. The Court adopted and articulated the theories about the relationship between the Constitution, citizenship, and the international laws of war that had been first developed out of the court in the early years of the war. These legal doctrines and understandings prevailed into the mid-twentieth century, until developments like the civil rights revolution and the increasing sense of judicial supremacy began to set the stage for today’s judicial management of the U.S. government’s relationship with military enemies under the aegis of the Constitution.